The main purpose of a motor is to convert electrical energy into mechanical energy. But when the motor\u2019s synchronous speed is less than the rotor speed, the motor acts like a generator and converts mechanical energy into electrical energy. This energy is typically fed back into capacitors in the DC bus, but the bus can only handle a limited amount of voltage before the drive registers a bus overvoltage fault. Regeneration gives this energy somewhere to go.<\/p>\n\n\n\n
The term \u201cregeneration\u201d was introduced into the general lexicon several decades ago with the mass production of hybrid electric vehicles (HEVs). In HEVs, energy generated during braking (regenerative energy) is used to charge the batteries.<\/em><\/p>\n\n\n\n Regenerative drives allow motors to produce\u00a0positive torque or negative torque, in either forward or reverse rotation. This is referred to as \u201cfour-quadrant\u201d operation and is depicted in the graphic below. Regeneration takes place in quadrants II and IV, where the direction of the torque opposes the direction of motor rotation (clockwise rotation with counterclockwise torque, or counterclockwise rotation with clockwise torque).<\/p>\n\n\n\n When the motor operates in one of these two modes, a regenerative drive can take the electrical energy generated by the motor and feed it back to the AC power source or to a common bus that supplies two or more drives. A common bus provides additional energy savings in that the power is only converted from AC to DC one time. When it is regenerated to the bus and passed to another inverter, it doesn\u2019t have to be converted. In either case\u2014whether it is fed back to the source or to a common bus\u2014the generated energy is used again, rather than being completely lost to heat, as is the case with a traditional dynamic brake.<\/p>\n\n\n\n
\n\n\n\nOne motor, four operating modes<\/h4>\n\n\n\n